Cortical activity is tightly regulated to support adaptive behavior, but the mechanisms underlying this regulation are unclear. In this project, we will investigate how cortical activity is regulated in vivo, directly at the site of synaptic input Dendrites actively process synaptic input using voltage-gated ion channels and NMDA receptors. We recently showed that these mechanisms support dendritic spiking in awake mice. These dendritic spikes propagate to the soma as depolarizations that can trigger conventional axonal spikes, and thus represent a layer of computational processing that contributes to neuronal selectivity. A recently elucidated circuit motif involving neuromodulation and dendrite-targeting interneurons could play a key role in regulating dendritic spiking during sensory processing and behavior. Here, we use dendritic patch clamp recordings, optogenetics, and new multiphoton imaging technology to interrogate this circuit motif, its effects on dendritic spiking, and its activity during sensory processing and behavior. Since dendritic spiking is an essential component of synaptic integration in cortical circuitry, and dysfunctional synaptic integration is implicated in complex psychiatric and neurological disorders, results from this project can eventually contribute to new therapeutic strategies.